Alkali metal reduction of alkali metal cations

Counter to synthetic convention and expectation provided by the relevant standard reduction potentials, the chloroberyllate, [{SiNDipp}BeClLi]2 [{SiNDipp} = {CH2SiMe2N(Dipp)}2; Dipp = 2,6-i-Pr2C6H3)], reacts with the group 1 elements (M = Na, K, Rb, Cs) to provide the respective heavier alkali metal analogues, [{SiNDipp}BeClM]2, through selective reduction of the Li+ cation. Whereas only [{SiNDipp}BeClRb]2 is amenable to reduction by potassium to its nearest lighter congener, these species may also be sequentially interconverted by treatment of [{SiNDipp}BeClM]2 by the successively heavier group 1 metal. A theoretical analysis combining density functional theory (DFT) with elemental thermochemistry is used to rationalise these observations, where consideration of the relevant enthalpies of atomisation of each alkali metal in its bulk metallic form proved crucial in accounting for experimental observations.


Sequential Reduction Reactions from Compound 6 to 10
A C6D6 solution of [{CH2SiMe2NDipp}2BeClLi]2 (6; 4 mg, 0.007 mmol) was added to an ampoule and vigorously stirred over a pre-formed sodium mirror (0.4 mg, 0.017 mmol) for 8 hours before being filtered into a Young's NMR tube, observing the formation of compound 7.The solution was then introduced to an ampoule and vigorously stirred over a pre-formed K mirror (6 mg, 0.15 mmol) for 8 hours before being filtered into a fresh Young's NMR tube, affording compound 8.The solution of 8 was then placed into a Young's NMR tube containing Rb metal and sonicated for 20 minutes, exclusively affording compound 9.The solution of 9 was then filtered into a vial containing Cs metal, agitated for 1 minute and immediately filtered into a Young's NMR tube observing the formation of compound 10.
The transformation of compound 7 to 8 can also be performed using KC8, though this could potentially necessitate a cation exchange rather than mutual alkali metal reduction.

Attempted Reduction of Compound 10 to 9
[{CH2SiMe2NDipp}2BeClCs]2 (10) in C6D6 (0.6 cm 3 ) was introduced to a Young's NMR tube containing Rb metal.The reaction mixture was sonicated for 20 minutes but only the observation of 10 was observed, even after sonicating for 1 hour, only compound 10 was observed.

Reduction of Compound 9 to 8
[{CH2SiMe2NDipp}2BeClRb]2 (9; 5 mg, 0.004 mmol) in C6D6 was introduced into an ampoule and vigorously stirred over a pre-formed K-mirror (6 mg, 0.15 mmol) for 4 hours before being filtered into a Young's NMR tube, observing the formation of compound 8.
The transformation of compound 9 to 8 can also be performed via a cation exchange reaction with KC8.

Attempted Reduction of Compound 8 to 7
[{CH2SiMe2NDipp}2BeClK]2 (8; 5 mg, 0.004 mmol) in C6D6 was introduced into an ampoule and vigorously stirred over a pre-formed Na-mirror (2 mg, 0.09 mmol) for 4 hours before being filtered into a Young's NMR tube.No evidence of reaction could be observed.
This reaction was also performed with 5 wt% Na/NaCl.Again, only the continued integrity 8 could be observed.

Crystallographic Details
Single Crystal X-ray diffraction data for compounds 7 -10 were collected on a SuperNova EosS2 diffractometer using CuKα (λ = 1.54184Å) radiation.The crystals were maintained at 150 K during data collection.Using Olex2, 4 the structures were solved with ShelXT and refined with the ShelXL 6 refinement package using Least Squares minimisation.Distance and ADP restraints pertaining to fractional-occupancy atoms were included on merit, throughout, to assist convergence.
The asymmetric unit in 7 contains one molecule of the complex and half of one molecule of hexane.C61 and C62 of the latter were treated for 70:30 disorder.
The hydrogen atoms attached to C26 have been located and refined, at a distance of 0.98 Å from the parent carbon, in the structure of 8.There is also one molecule of benzene present, per asymmetric unit.
In 9, the asymmetric unit comprises two crystallographically independent dimers plus one molecule of benzene.Both the rubidium centre and the isopropyl group based on C25 were treated for 60:40 disorder.The ADP pertaining to Cl1 is also suggestive of some disorder, but efforts to model same did not converge well without excessive restraints and, hence, this was abandoned.In the fragment based on Be2, the halide was successfully modelled to take account of 60:40 disorder.
In 10 there is half of a dimer molecule in the asymmetric unit plus one benzene molecule.The remainder of the complex is generated via the inversion symmetry intrinsic to space group P21/n.C15 and C16 were treated for 65:35 disorder.

Computational Details
C1. DFT calculations were performed with Gaussian 16 (A.03). 7The Na, Si, Cl, K, Rb and Cs centres were described with the Stuttgart relativistic effective core potentials (RECPs) and associated basis sets, 8 and the 6-31G** basis set was used for all other atoms (BS1). 9A polarization function was also added to Si (ζd = 0.284), Cl (ζd = 0.640), K (ζd = 1.000),Rb (ζd = 0.491) and Cs (ζd = 0.306). 10Initial BP86 optimizations were performed using the 'grid = ultrafine' option, 11 with all stationary points being fully characterized via analytical frequency calculations as minima with all positive eigenvalues.(Note, that for the structures of (C6H6)Li the presence of small imaginary modes (> -10 cm -1 ) persisted despite attempts to remove them, including re-optimization with 'very tight' convergence criteria and moving the geometry with respect to the imaginary modes).All energies were recomputed with a larger basis set featuring 6-311++G** basis sets on all atoms (BS2), except for Rb and Cs, where the Stuttgart RECPs were again employed with the aforementioned additional polarization functions.Corrections for the effect of benzene (ε = 2.2706) solvent were introduced using the polarizable continuum model and BS1. 12 Single-point dispersion corrections to the BP86 results employed Grimme's D3 parameter set with Becke-Johnson damping as implemented in Gaussian (D3BJ). 13Further single point calculations were performed for the basis set testing.These include; def2-SVPX for all atoms (including the corresponding ECPs) 14 def2-TZVPPX, Sapporo-DZ-2012X, 15 Sapporo-TZ-2012 (however, both Sapporo basis sets are not available for Cs).Additional single point calculations were performed with ORCA 5.0.1, 16 using the ZORA-def2-TZVPP for all atoms, except Rb and Cs, for which the all-electron ZORA(-SARC-)TZVPP basis set was used, 17 including CPCM solvation, D3BJ dispersion, and the ZORA relativistic approach. 18

C2. Basis Set Testing of M + •••C6H6 Interactions.
Basis set testing was carried out to discern whether an effective core potential or an all-electron basis set was more appropriate to describe the alkali metal centres in the [Be]2M2 structures, 7 -10, resulting from reductive de-lithiation of 6.Given that all resulting complexes are considered to feature stabilizing cation-arene interactions with the group 1 metal centres, we considered it appropriate to benchmark against a series of [M(C6H6)n] + systems (n = 1 -2), the experimental bond dissociation energies (BDEs, ΔE) of which have been measured by gas-phase collision-induced dissociation (CID) experiments, and reported by Armentrout and co-workers. 19In their study, Armentrout identified that the use of RECPs to describe the heavier group 1 elements led to divergent BDEs with respect to experimental data.The results of the basis set testing of these systems, at both the BP86-D3BJ/BS2//BP86/BS1 and BP86-D3BJ/ZORA(-SARC)-def2-TZVPP//BP86/BS1 levels of theory (simply expressed as 'ZORA' in Supplementary Table 2), are presented below in Supplementary Table 2.

Table 1 :
Crystal data and structure refinement for compounds 7 -10.